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Version: 29.0
important

ts-jest will take into account of the following things when working with ESM:

  • Jest Runtime
  • Check type: "module" in package.json ONLY WHEN module in tsconfig has hybrid value: either Node16/Node18/NodeNext
  • When module in tsconfig isn't set to a hybrid value, module MUST HAVE one of the ES values, e.g. ES2015, ES2020 etc...

References

Configure Jest runtime​

Check ESM Jest documentation.

info

Jest runtime currently has a few issues related to support ESM:

Overall progress and discussion can be found at https://github.com/jestjs/jest/issues/9430

Configure tsconfig​

Using ES module values​

// tsconfig.spec.json
{
  "compilerOptions": {
    "module": "ESNext", // or any values starting with "es" or "ES"
    "target": "ESNext",
    "esModuleInterop": true
  }
}

Using hybrid module values​

important

Hybrid module values requires type field in package.json to be set explicitly to:

  • commonjs for CommonJS code
  • module for ESM code
// tsconfig.spec.json
{
  "compilerOptions": {
    "module": "Node16", // or Node18/NodeNext
    "target": "ESNext",
    "esModuleInterop": true
  }
}

Configure Jest config​

Configure your Jest configuration to use one of the utility functions

Example​

// jest.config.ts
import type { Config } from 'jest'

const jestConfig: Config = {
  // [...]
  preset: 'ts-jest/presets/default-esm', // or other ESM presets
  moduleNameMapper: {
    '^(\\.{1,2}/.*)\\.js$': '$1',
  },
  transform: {
    // '^.+\\.[tj]sx?$' to process ts,js,tsx,jsx with `ts-jest`
    // '^.+\\.m?[tj]sx?$' to process ts,js,tsx,jsx,mts,mjs,mtsx,mjsx with `ts-jest`
    '^.+\\.tsx?$': [
      'ts-jest',
      {
        useESM: true,
      },
    ],
  },
}

export default jestConfig

Resolve .mjs/.mts extensions​

To work with .mts extension, besides the requirement to run Jest and ts-jest in ESM mode, there are a few extra requirements to be met:

  • package.json should contain "type": "module"
  • A custom Jest resolver to resolve .mjs extension, for example:
import type { SyncResolver } from 'jest-resolve'

const mjsResolver: SyncResolver = (path, options) => {
  const mjsExtRegex = /\.mjs$/i
  const resolver = options.defaultResolver
  if (mjsExtRegex.test(path)) {
    try {
      return resolver(path.replace(mjsExtRegex, '.mts'), options)
    } catch {
      // use default resolver
    }
  }

  return resolver(path, options)
}

export = mjsResolver